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911 SC motor Induction noise?

Engine

Vehicle Information: Model: 911S; Year: 1977; Body Type: Coupe; Total Mileage: 166000 (chassis) 70000(engine);

I realize this may be impossible to diagnose by email, but here goes: My 77 911S has a 3.0L SC engine enlarged to 3.2L. It has an early distributor with points, SSI heat exchangers and the CIS from the previous 2.7L engine. I have a new 1980 WUR but no lambda control. The car was rearended last year and repaired by one of the best body shops in Toronto, using a frame machine, and my regular mechanic took care of mechanicals. The muffler was replaced along with the sound absorbtion pad and engine cover. All other work was to the body/chassis. Since the repairs, there is a somewhat different sound coming from the engine area while at highway speeds. While coasting, the engine note is very typical 911. When I apply throttle, there is an audible "growl" or "roar", which goes away as soon as I let off the gas. The car accelerates as it should (in fact, with the engine mods the car is quite quick), the engine revs and idles as it should, and everything seems fine except for the noise and a little bit of popoff valve popping when starting from cold. The noise occurs at speeds from 50mph on up, and in 3rd, 4th or 5th gears. Driveability is very good, and there are no other unusual noises, smells, smoke or overheating. Am I just hearing some induction noise, or something associated with the new (Dansk) muffler? Or could it be something seriously wrong like a cracked airbox or some fuel system related problem? Thank you.

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